cgofuse: add file table realloc/shrinking method

internal/filesystem/cgofuse/table.go

@@ -70,6 +70,38 @@ func (files handleSlice) extend() (handleSlice, error) {
 	return newTable, nil
 }
 
+func (files handleSlice) shrink(lowerBound int) handleSlice {
+	var (
+		emptySlots int
+		filesLen   = len(files)
+		filesCap   = cap(files)
+	)
+	for i := filesLen - 1; i != -1; i-- {
+		if files[i] != nil {
+			break
+		}
+		emptySlots++
+	}
+	var (
+		newLen = filesLen - emptySlots
+		bound  = boundCheck(lowerBound, newLen)
+	)
+	if newLen == bound || filesCap == bound {
+		return nil
+	}
+	newTable := make(handleSlice, newLen, bound)
+	copy(newTable, files)
+	return newTable
+}
+
+func boundCheck(lowerBound, oldCap int) int {
+	newCap := lowerBound
+	for newCap < oldCap {
+		newCap *= 2
+	}
+	return newCap
+}
+
 func (ft *fileTable) add(f fs.File) (fileDescriptor, error) {
 	ft.Lock()
 	defer ft.Unlock()